A wide variety of sensors, including temperature sensors, have been developed using fiber optics. In a great many of these sensors, the length of the optical fiber serves only as a transmission means and not as the sensor itself, the sensor typically being separate from the optical fiber or located only at the tip of the fiber. Such sensors are referred to here as point sensors. The sensor may or may not involve light phenomena. For example, U.S. Pat. No. 4,848,923 (Jul. 18, 1989) to Ziegler et al. uses a fiber/optic to transmit light pulses that are obtained by converting electrical pulses from a pulse generator wherein the pulse gaps change in relation to an environmental change such as temperature. Here the sensor involves electrical pulses not light phenomena.
In some instances, the point sensor is not a temperature sensor; rather compensation must be made for temperature changes that may cause erroneous readings. For example, U.S. Pat. No. 4,487,206 (Dec. 11, 1984) to Aagard is a point optical pressure sensor that uses fiber optics to carry transmitted light to the end of the fiber and two optical fibers to return reflected light to a detector, one fiber to return light from a reflecting pressure sensitive lens-diaphragm-mirror combination and the second fiber to return reflected reference light. Such an arrangement compensates for temperature and fiber bending changes. U.S. Pat. No. 4,613,811 (Sep. 23, 1986) to Vaerewyck et al. discloses a point magneto-optical current sensor that uses a fiber optic to transmit and receive a light signal that is compensated for temperature, loop degradation, and linearity in a Faraday current sensor. U.S. Pat. No. 4,724,316 (Feb. 9, 1988) to Morton discloses a point fiber optic sensor that depends on changes in the curvature of a fiber optic wave guide caused by a mechanical force, e.g. a cam, to produce variations in light intensity in the fiber optic. Coating materials and support members are provided so as to minimize thermal stress. U.S. Pat. No. 4,639,138 (Jan. 27, 1987) to Martin et al. is a fiber-optic, rotation-rate, point sensor that uses two optic fiber loops to eliminate temperature-dependent index of refraction and fiber length sensitivity.
There are many point sensors for the detection of temperature. The optical fibers involved (other than at the tip or end of the fiber) serve only to conduct light signals and are not a part of the sensor. For example, U.S. Pat. No. 4,574,172 (Mar. 4, 1986) to Burack et al. uses an optical cable as a radiation input source for a two color pyrometer that is used to determine and control brazing temperature. U.S. Pat. No. 4,672,199 (Jun. 9, 1987) and 4,758,087 (Oct. 27, 1987) to Anderson et al. uses an optical fiber to conduct light to a sensor with a reflective surface responsive to temperature or pressure changes. Bimetallic strips are used for temperature sensing.
U.S. Pat. No. 4,758,087 (Jul. 19, 1988) to Hicks, Jr. uses a resonant cavity formed from an optical fiber segment and coupled to an input and output fiber to measure external conditions such as temperature and pressure. U.S. Pat. No. 4,749,254 (Jun. 7, 1988) to Seaver discloses an optical point sensor that relies on an optical filter to detect the wavelength shift of the band edges of various optical and infrared filters with changes in temperature, pressure and index of refraction of the environment. The sensor uses an optical fiber transmission link. U.S. Pat. No. 4,883,062 (Nov. 28, 1989) to Nicholson shows a point detector consisting of an interference edge filter mounted on the end of an optical fiber. The transition slope of the edge filter curves shifts depending on changes in the parameter measured. Reflected light from the edge filter is passed through an interference bandpass filter that has an edge slope intersecting the slope of the edge filter. Shifts in the slope of the edge filter, due to measured parameter variation, result in a change in the intensity of light passing to a detector. U.S. Pat. No. 4,703,175 (Oct. 27, 1987) to Salour et al. discloses a point fiber optic sensor in which light is transmitted through semiconductor material that absorbs a portion of the light as a function of temperature. U.S. Pat. No. 4,790,669 (Dec. 13, 1988) to Christensen also uses a semiconductor material at the end of an optical fiber to determine changes in reflected light caused by temperature changes in the semiconductor material.
U.S. Pat. No. 4,714,342 (Dec. 22, 1987) to Jackson et al. is a temperature point detector that measures the interference pattern of reflected signal and reference beams. Thermal expansion of the sensor fiber produces a dimensional change in the fiber giving rise to a phase shift between the reference and signal beams. U.S. Pat. No. 4,708,494 (Nov. 24, 1987) to Kleinerman uses a point detector with a luminescent material that has a temperature-dependent absorption coefficient. Excitation light is directed to the sensor through an optical fiber. The resulting luminescence light intensity is a function of temperature.
U.S. Pat. No. 4,750,139 (Jun. 7, 1989) to Dils discloses a temperature sensor that uses a black body emitter to emit light with a flux density proportional to temperature. The light is transmitted through optical fibers to a detection system. An optical light pipe or a pyrometer may also be used as the input for the system. U.S. Pat. No. 4,794,619 to Tregay discloses an optical temperature probe that uses the thermal emission from a recess in the tip of an optical temperature detection device. U.S. Pat. No. 4,859,079 (Aug. 22, 1989) to Wickersheim et al. uses a blackbody sensor to direct radiation to an infrared sensor that is in communication with an optical sensor that uses a luminescent material. Light is directed to the luminescent material from a detector and the temperature modified light is than returned to the detector using an optical fiber. U.S. Pat. No. 4,752,141 (Jun. 21, 1988) to Sun et al. also uses a sensor having luminescent material that is excited by light carried along an optical fiber with the resultant luminescent radiation being returned to a detector along the optical fiber. U.S. Pat. No. 4,459,044 (Jul. 10, 1984) to Alves uses a temperature sensitive phosphor material at the end of an optical fiber to detect temperature changes by passing ultraviolet radiation from a source through the optical fiber to the phosphor and then detecting the visible radiation returned from the phosphor through the optical fiber. U.S. Pat. No. 4,789,992 (Dec. 6, 1988) to Wickersheim et al. uses a luminescent material placed on the surface of an object whose temperature is to be measured. The luminescent material is excited by light from an optical fiber and the resultant luminescent radiation is returned to a detector through the optical fiber. U.S. Pat. No. 4,223,226 (Sep. 16, 1980) to Quick et al. uses a phosphor material at the end of an optical fiber to detect temperature change. The phosphor receives incident light stimulation from an optical fiber and the emitted phosphorescent radiation, of which the amplitude decay rate and wavelength are functions of temperature, is transmitted to a detector through the input fiber or a separate optical fiber. U.S. Pat. No. 4,749,856 (Jun. 7, 1988) to Walker et al. is a point sensor that relies on the changes in optical transmission properties of polymeric materials in response to changes in temperature, humidity, pressure, sound, etc.
By inserting one or more point sensors along the optical fiber it is possible to obtain measurement of the physical property at the point of location of the sensor along the optical fiber. Such intermittent sensors are termed "quasi-distributed" sensors. For example, U.S. Pat. No. 4,201,446 (May 6, 1980) to Geddes et al. uses a short section of a temperature-sensitive liquid core or liquid cladding optical fiber inserted in series with a conventional optical fiber to determine temperature based on a reduction in the intensity or numerical aperture after the light passes through the temperature-sensitive liquid region. U.S. Pat. No. 4,861,979 (Aug. 29, 1989) to Tardy et al. is a quasi-distributed sensor in which a gap is introduced at successive points along the length of the fiber, i.e., the fiber becomes a series of discrete segments with a gap between each segment. The gap is filed with a medium that is sensitive to the physical parameter being measured, e.g., temperature, such that the coefficient of reflection from the end (optical surface) of each segment and the intensity of a returned light pulse depend on changes in the gap medium. An appropriate reception circuit allows measurement of the physical parameter at each gap along the fiber.
Another methodology for measuring temperature is what is termed "distributed" fiber sensing in which the parameter to be measured can be determined along the entire length of the fiber. In one form of distributed fiber sensing, termed intrinsic fiber sensing, properties of the fiber itself are used to detect various physical parameters including temperature as they effect a portion or all of a fiber along its length. For example, U.S. Pat. No. 4,362,057 (Dec. 7, 1982) to Gottlieb et al. takes advantage of the fact that when a fiber is heated, the thermal radiation emitted will depend on the temperature, emissivity of the fiber material, and the spectral range of the wavelength being observed. Both the temperature and position of the hot spot along the fiber can be determined. U.S. Pat. No. 4,295,739 (Oct. 20, 1981) to Meltz et al. uses a multicore optical fiber to determine temperature or strain changes along the length of the fiber as a result of cross-talk between adjacent cores in the fiber. U.S. Pat. No. 4,647,203 (Mar. 3, 1987) to Jones et al. uses an optical fiber Fabrey-Perot interferometer that is sensitive to any parameter that influences the optical fiber's length, e.g., temperature, magneto-strictive effects, etc. U.S. Pat. No. 4,659,923 (Apr. 21, 1987) to Hicks, Jr. uses a dual path optical fiber to sense a relative change in the propagation constant as a result of an applied force that produces an interference variation. The applied force can be a change in temperature or pressure. U.S. Pat. No. 4,830,513 (May 16, 1989) to Grego uses the backscattered radiation from the fiber core itself to measure temperature variations along the fiber. The use of an optical time domain reflectometer allows the temperature information to be associated with the backscattering point along the fiber. The methodology is based on frequency spectrum variations in the backscattered radiation from the fiber core with respect to incident radiation.
The concept of distributed fiber sensing has also been discussed in the non-patent literature. The most active area of distributed fiber optic sensing has been temperature measurement with at least one review article having appeared on the topic. A. J. Rogers, "Distributed Optical-Fibre Sensors", J. Phys. D: Appl. Phys. 19, 2237-55 (1986). The most practical of the distributed sensor concepts use Optical Time Domain Reflectometry (OTDR) to monitor backscattered light from segments of the fiber along its entire length.
For the OTDR approach, the distance of any particular change in the backscattered light can be calculated by measuring the elapsed time of the returned pulse. If the time required to propagate back and forth is .tau., then the location, L, of the change is given by ##EQU1## where c=velocity of light in vacuum (3.times.10.sup.8 m/s), and
n=refractive index of the fiber. PA0 .lambda.=wavelength of incident light source and PA0 a=radius of fiber core.
To use the OTDR approach as the basis for a distributed fiber optic sensor, the key is to modify an ordinary optical fiber along its entire length so that the modification results in a fiber where the local loss or backscattering characteristics or both are changed by changes in a particular parameter, e.g., temperature. By monitoring the amount of change in the local backscattered light characteristics using the OTDR methodology, environmental changes can be measured along the entire fiber length. Furthermore, the point (in time) of the maximum or minimum OTDR signal change can be used to identify the location (in space) of the maximum or minimum parameter value.
The OTDR signal consists of light backscattered during the progress of a pulse traveling down the fiber sensor. The amount of backscattered light, P.sub.bs (l), from a given location l along the fiber, within the scattering element dx, can be written as ##EQU2## where P.sub.o is the power launched into the fiber, .DELTA.t the source pulse width, and .nu..sub.g the pulse group velocity. The term NA in Equation (2) is the fiber's local numerical aperture (i.e., light capturing efficiency), which is dependent on the index of refraction of the core and clad materials. In Equation (2), C.sub.s and .alpha. are the scattering constant and the total loss coefficient, respectively.
In order to use the backscattered light pulse intensity in a distributed temperature sensor, it is clear from Equation (2) that .alpha., C.sub.s or NA must be dominant functions of temperature.
Based on the theory of D. Gloge, "Weakly Guiding Fibers", Applied Optics, 10 (10), 2252-58 (1971), C.sub.s and .alpha. can be written as follows: EQU C.sub.s =(.alpha..sub.R).sub.co +(.alpha..sub.s).sub.co +P.sub.cl /P.sub.t (.alpha..sub.s).sub.cl ( 3) EQU .alpha..perspectiveto..alpha..sub.co +P.sub.cl /P.sub.T (.alpha..sub.cl).(4)
In Equation (3) and (4), .alpha..sub.R is the Raman scattering coefficient and .alpha..sub.s is the Rayleigh scattering coefficient. Subscripts co and cl are associated with the core and cladding, respectively. The factor P.sub.cl /P.sub.T is the fraction of propagating power that exists in the cladding due to evanescent wave effects.
Researchers at Southampton University, M. C. Farriers and M. E. Fermann, "Temperature Sensing by Thermally Induced Absorption in a Neodymium Doped Optical Fibre", Proceedings of SPIE, Fiber Optic Sensors, The Hague, Netherlands, Vol. 798, 115-117 (1987), produced a distributed temperature sensor based on temperature attenuation of the core glass, i.e., (.alpha..sub.s).sub.co in Eq. (3). The fiber core was doped with neodymium and a 904 nm pulsed laser source was used to obtain distributed temperature sensor data. At this wavelength, the temperature dependent core absorption .alpha..sub.co (T) was found to be linear over a 50.degree. C. increment. Although the doped-fiber concept appears feasible, doping non-uniformity problems and the need to produce special fiber preforms prior to sensor fabrication appear to have dampened interest in this methodology.
The first demonstration of a distributed fiber optic sensor used a special fiber having a liquid core, A. H. Hartog, "A distributed Temperature Sensor Based on Liquid-Core Optical Fibers", J. of Lightwave Tech. LT-1, 498-509 (1983). In this case, increasing the fiber temperature causes an increase in the scattering coefficient of the liquid core, (.alpha..sub.s).sub.co, by 0.02 dB/.degree. C. for the length of fiber tested. Although workable, the liquid core fibers have not been accepted since producing the sensor requires a large deviation from standard fiber manufacturing technology.
Scattering in optical fibers is caused principally by the Rayleigh effect which results from inhomogeneities in the core glass that are formed in the production process. This type of scattering is at the same wavelength (elastic) as the incident light and core scattering is largely independent of temperature changes (except for liquid core fibers described). However and as noted above, Grego (U.S. Pat. No. 4,830,513) has taken advantage of this intrinsic property by comparing frequency spectrum variations between backscattered and incident radiation due to temperature variation.
There is a small (100-1000 times less) contribution to the scattered power from the Raman effect, .alpha..sub.R in Equation (3), which originates from molecular and crystalline effects within the core glass. A key attribute of the Raman effect is that it results in temperature-dependent scattering at a different wavelength than the incident light, i.e., inelastic. Raman scattering has received the most attention as a method of providing a distributed fiber optic temperature sensing (DFOTS) system. Commercial devices are being supplied by several companies.
Initial Raman work was carried out in Great Britain which led to a practical device incorporating solid state components, J. P. Dakin, et al., "Distributed Optical Fiber Raman Temperature Sensor Using a Semiconductor Light Source and Detector", Elect. Lett., 21 (13), 569-570 (1985). In this case, the ratio of Raman Stokes to anti-Stokes scattering intensity was used to determine temperature along the sensor length independent of attenuation losses. York Technology, A. H. Hartog, et al., "Distributed Temperature Sensing in Solid Core Fibers", Elec. Lett, 21 (32), 1061-1062 (1985), measured the anti-Stokes backscattered light from the fiber and used a different referencing scheme to cancel attenuation effects.
Efforts have recently been made to reduce the spatial resolution to about 1 meter using narrow pulsed laser sources. K. Ogawa, et al., "A Fiber-Optic Distributed Temperature Sensor With High-Distance Resolution", Springer Proceedings in Physia, Vol 44, Optical Fiber Sensors, Springer-Verlag Berlin (1989). However, the fact that Raman scattering produces such a weak signal limits resolution capability.
Equations (2) through (4) indicate that for negligible cladding attenuation or scattering effects, and constant core scattering and attenuation properties, the change in NA with temperature can be used to form a DFOTS system. In this case, Equation (2) becomes EQU P.sub.bs (l).perspectiveto.NA(T) exp (-2al) (5)
where ##EQU3## and EQU .DELTA.n=n.sub.co -n.sub.cl. (7)
From Equation (6) and (7), it can be seen that the proper choice of core and cladding materials, such that .DELTA.n decreases with increasing temperature, will result in a decreasing NA(T) and P.sub.bs (l). This is accomplished by noting the index change with temperature, dn/dT, of various materials and using these special materials (glasses or polymers) to provide the temperature sensor.
A changing NA approach has been suggested for providing a DFOTS Systems. However, the technique does not appear to be practical since only special material selections will provide the proper combination of refractive index changes. Such special material selections can introduce other problems such as fiber pulling irregularities. Sensor cost can be high if nonstandard glasses are used.
Since there is evanescent wave penetration of guided light into the cladding, low-loss clad material is normally chosen to keep the overall fiber loss coefficient (.alpha.) as small as possible. The clad contribution to overall loss is shown in Equation (4) as P.sub.cl /P.sub.T (.alpha..sub.cl) where .alpha..sub.cl is the clad loss coefficient and P.sub.cl /P.sub.T is the fraction of power traveling in the clad material. D. Gloge, "Weakly Guiding Fibers", Applied Optics, 10 (10), 2252-58 (1971), has shown that P.sub.cl /P.sub.T term can be written as: EQU P.sub.cl /P.sub.T =2 .lambda./[3.pi.a(2n.sub.co .DELTA.n(T)).sup.1/2 ](8)
where
As the core or clad index changes with temperature, such that .DELTA.n(T) changes, more or less power is present in the cladding. Therefore, more or less fiber transmission loss will occur according to Equation (4).
The clad attenuation DFOTS technique was described theoretically by M. Gottlieb and G. B. Brandt, "Temperature Sensing in Optical Fibers Using Cladding and Jacket Loss Effects", Appl. Opt. 20, 3867-73 (1981), and is the subject of U.S. application 059,545 assigned to the same assignee as the present invention, and which is herein incorporated by reference.